1. Field of the Invention
The present invention relates generally to positioning devices, and, more specifically, to an apparatus for positioning an end effector about multiple degrees of freedom. The apparatus and associated controller is especially suited for positioning an end portion of a fiber optic cable when used in a free-space optical communications system.
2. Background Information
With the increasing popularity of wide area networks (WANs), such as the Internet and/or the World Wide Web, network growth and traffic has exploded in recent years. Network users continue to demand faster networks and more access for both businesses and consumers. As network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to present day hardwired or fiber network solutions is the use of wireless optical communications. Wireless optical communications utilize point-to-point communications through free-space and therefore do not require the routing of cables or fibers between locations. Thus, wireless optical communications are also known as free-space or atmospheric optical communications. For instance, in a free-space optical communication system, a beam of light is directed through free-space from a transmitter at a first location to a receiver at a second location. Data or information is encoded into the beam of light, and therefore, the information is transmitted through free-space from the first location to the second location.
An important aspect of a free-space optical communications system is tracking. In particular, it is important that the optical communications beam (e.g., laser beam) is aimed properly from the transmitter at the first location and that the receiver at the second location is aligned properly to receive the optical communications beam. For example, assume that a transmitter is mounted on a first building and that a receiver is mounted on a different second building. Assume further that there is a line of sight between the transmitter and receiver. It is important for the transmitter on the first building to be configured to accurately direct or aim the optical communications beam at the receiver on the second building.
Tracking is utilized for maintaining the alignment of the optical communications beam between the transmitter and receiver in various situations or disturbances. Examples of these various situations or disturbances include the swaying of the buildings due to for example windy conditions, vibration of the platforms on which the transmitter and/or receiver are mounted, atmosphere-induced beam steering, etc. If the tracking system is unable to compensate for disturbances, the optical communications beam is no longer properly aimed at the receiver and, consequently, communications between the transmitter and receiver are lost or impaired.
The present invention provides an apparatus for positioning an end effector and components attached thereto and an associated multi-channel position controller. The positioner apparatus comprises a plurality of bimorph members, generally elongated in shape, that are coupled end-to-end. For example, in one embodiment, four bimorph members are coupled end-to-end, wherein each adjacent pair of bimorph members are substantially orthogonal. Each bimorph member includes a pair of elongated piezoelectric bimorph elements that cause a localized deformation in the bimorph member when driven with an input voltage. Accordingly, each bimorph member can be caused to bend inwardly or outwardly, as well as twist, depending on its drive voltages. In one configuration, four substantially orthogonal bimorph members are connected end-to-end, with the last bimorph member operatively coupled to the end effector. By controlling the drive voltages to each of the piezoelectric bimorph elements, the end effector may be moved about five degrees of freedom, including displacement along orthogonal X, Y and Z axes, and rotations about elevation and azimuth axes.
The multi-channel position controller implements a pair of correlative feedback loops for each channel, including a global feedback signal that is shared by all channels and a local feedback signal corresponding to each individual channel. In one embodiment, the multi-channel position controller is implemented in a free-space optical communications system, wherein the global feedback corresponds to a signal strength measurement of a transmitted signal, and the local feedback signal comprise a position sense signal that is produced by each piezoelectric bimorph element. The positioner is correlative in the sense that it uses both feedback signals to correlate which direction the drive voltage input for a given channel needs to change.